APS are solid state imagers where each pixel contains a photosensing means, charge to voltage conversion means, reset means and all or part of an amplifier. APS devices are typically operated in a manner wherein each line, or row, of the imager is integrated, read out and then reset at a different time interval. Therefore, in reading out the entire image, each line has captured the scene at a different point in time. Since illumination conditions vary temporally, and since objects in the scene may also be moving, this method of read out can produce line artifacts in the resulting representation of the image. This limits the usefulness of APS devices in applications where high quality motion or still images are required.
Additionally, the charge to voltage conversion region, and other active transistor regions (i.e. other than the photosensing region), of APS devices are not shielded from the scene illumination. As a result, free electrons will be generated in those regions. These electrons are not effectively confined to the pixel in which they were generated, and can diffuse into adjacent pixels. This causes a degradation of the spatial accuracy of the image signal, and the modulation transfer function (MTF) of the image sensor. This is especially problematic in color image sensors where this pixel cross-talk leads to color mixing, adversely affecting the color balance of the image.
Since APS devices are typically fabricated in CMOS foundries, they do not incorporate color filter arrays (CFA) or micro-lens arrays (.mu.Lens), and the shape and size of the photosensing area has not been optimized for incorporation of CFA and .mu.Lens. A reason for this is that prior art active pixel sensors are typically monochrome. For most imaging applications it is desirable to have a color sensor. Even if CFA and .mu.Lens were placed on prior art APS devices, the resulting images would have poor color MTF due to cross-talk and photosensing area that has not been optimized.
To solve the above discussed problems, it is desirable to perform integration at the same point and interval of time for every pixel, and subsequently transfer this charge to a storage region in each pixel that is shielded from the scene illumination. This is referred to as frame integration. It is also desirable to have all regions except for the photodetector effectively shielded from the scene illumination to improve the MTF. It is further desirable to provide blooming control during integration, and more importantly for frame integration, during storage and read out of the device. Finally, it is desirable to incorporate CFA and .mu.Lens, and design the photosensing region to enable effective use of CFA and .mu.Lens. These and other issues are solved by the teachings of the present invention.